This invention relates to metal powder compositions and, in particular, to metal powder compositions in which an iron-based powder is admixed with a minority fraction of a ferrite powder having a smaller particle size distribution, and to composite structures formed from the metal powder composition.
Manufacturers have begun to utilize powder metallurgy techniques to fabricate magnetic core components for alternating-current applications as replacements for core components made from laminated steel sheet. To that end, iron-based powders have been compressed by conventional powder metallurgy pressing techniques to produce a high density metallurgical compact for use as ignition cores, transformer cores, and stators and rotors for alternating-current and direct-current motors and generators. When judged solely by a manufacturing criterion, powder metallurgy techniques are efficient for producing core components having a desired net-shape or near to net-shape. However, metallurgical compacts formed from conventional iron-based powders by powder metallurgy techniques lack certain electromagnetic and physical properties that are highly desired in alternating-current applications.
When exposed to a rapidly varying electromagnetic field in alternating-current applications, core components formed of conventional iron-based powders are subject to undesirable hysteresis losses and eddy current losses. The eddy currents represent flows of electrical energy in the core component and opposed the desired flow. Eddy currents are converted into heat, which results in overheating and lowered efficiency of the core component. When compared with core components of laminated steel sheet, powder metal core components tend to exhibit inferior properties at frequencies less than about 500 Hz, such as significantly higher core losses
To reduce the undesirable core and hysteresis losses, a uniform layer of one or more dielectric materials may be associated with the exterior of the particles of iron-based powder. Following compaction, the dielectric material electrically insulates individual particles from other adjacent particles. Because the insulation provided by the dielectric material persists following compaction, core losses are reduced in the magnetic core component. However, sintering has been found to degrade the interparticle insulation. Therefore, following compaction, green compacts formed from dielectric covered iron-based powders are usually not sintered or otherwise heated to a temperature sufficient to degrade the dielectric coating.
Among the most accepted dielectric materials are thermoplastic polymers. However, the presence of the polymer in the green compact significantly reduces the flux-carrying capacity and the permeability of the overall component to unacceptable levels for use as core components in certain alternating-current applications. In other alternating-current applications, green compacts formed from conventional iron-based powder compositions may lack the desired electrical resistivity because the polymer coating does not adequately separate adjacent particles of iron-based powder.
Thus, an iron-based powder composition that can be compacted by powder metallurgy techniques is needed for producing a green compact having acceptable physical properties, such as compact strength, and electromagnetic properties, such as permeability and resistivity.
The present invention provides a powder metal composition comprising an admixture of a collection of iron-based particles having a first particle size distribution and a collection of ferrite particles having a second particle size distribution. The ferrite particles constitute about 0.1 to about 10 percent by weight of the total weight of the admixture. The particle size distribution of the ferrite particles is generally less than the particle size distribution of the iron-based particles. In one embodiment, the iron-based powder have may a particle size distribution ranging between about 45 and about 500 microns and the ferrite particles have a particle size distribution ranging between about 1 and about 45 microns. To promote an association between the particles of the ferrite powder and the particles of iron-based powder, the iron-based powder may be coated with a binding agent, such as a thermoplastic polyacrylate, which encourages the ferrite particles to adhere to the exterior of the particles of the iron-based powder.
The presence of ferrite particles dispersed between adjacent iron-based particles increases the bulk resistivity of the compacted composite structure. The increased resistivity significantly reduces overall eddy current losses, which is particularly important for high frequency and low induction applications. The presence of the ferrite particles also fortifies the permeability of the compacted composite structure.